In many aircraft, fuel is not only supplied to the gas turbine propulsion engines to propel the aircraft, but is also used for thermal management. That is, a portion of the fuel withdrawn from the fuel tanks may also be used to cool various components and lubrication oils, and then returned to the fuel tanks. The fuel in the aircraft fuel tanks is typically relatively cool (e.g., ambient temperature), whereas the fuel that has been withdrawn from the fuel tanks, pressurized by one or more pumps, directed through various fuel/oil coolers, equipment cooling circuits, fuel control bypass loops, and the like, is relatively hot (e.g., about 325° F.).
When the propulsion engines are operating at relatively low power conditions, burn flow rate of fuel may also be relatively low. In some instances, the relatively low burn flow rate may cause the fuel that is returned to the fuel tanks to be at a relatively high temperature. Regulatory authorities place limits on the temperature of fuel being returned to an aircraft fuel tank. Thus, some aircraft fuel supply systems include a plurality of valves to selectively mix relatively cool fuel with the relatively hot return fuel to lower its temperature before being returned to the fuel tank(s). Typically, to ensure proper operation across the operating range, both the mix of hot and cool fuel flows and the total fuel flow amount are accurately controlled to two different levels. Presently known configurations rely on a plurality of solenoids, which are used to position a metering valve to achieve the two required flow levels and to position a separate shut-off valve to achieve shutoff. This configuration also relies on two position sensors to enable valve position control and verification.
The presently known configurations for selectively controlling return fuel flow to a fuel tank are generally safe, reliable, and robust, but do exhibit certain drawbacks. For example, these configurations rely on multiple valves, multiple solenoids, and multiple position sensors, which can increase overall system cost and weight.
Hence, there is a need for a device that selectively controls return fuel flow to a fuel tank that does not rely on multiple valves, multiple solenoids, and multiple position sensors. The present invention addresses at least this need.